Messages in this thread | | | Date | Mon, 1 Feb 2016 17:06:25 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler |
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Honestly, I am not sure I understand what this patch does and why, and it is white space damaged, please fix.
On 01/31, Stas Sergeev wrote: > > linux implements the sigaltstack() in a way that makes it impossible to > use with swapcontext(). Per the man page, sigaltstack is allowed to return > EPERM if the process is altering its sigaltstack while running on > sigaltstack. > This is likely needed to consistently return oss->ss_flags, that indicates > whether the process is being on sigaltstack or not. > Unfortunately, linux takes that permission to return EPERM too literally: > it returns EPERM even if you don't want to change to another sigaltstack, > but only want to temporarily disable sigaltstack with SS_DISABLE. > You can't use swapcontext() without disabling sigaltstack first, or the > stack will be re-used and overwritten by a subsequent signal.
So iiuc you want to switch the stack from the signal handler running on the alt stack, and you need to ensure that another SA_ONSTACK signal won't corrupt the alt stack in between, right?
Perhaps you can update the changelog to explain why do we want this change.
> @@ -2550,8 +2551,11 @@ static inline int sas_ss_flags(unsigned long sp) > { > if (!current->sas_ss_size) > return SS_DISABLE; > - > - return on_sig_stack(sp) ? SS_ONSTACK : 0; > + if (on_sig_stack(sp)) > + return SS_ONSTACK; > + if (current->sas_ss_flags == SS_DISABLE) > + return SS_DISABLE; > + return 0;
So this always return SS_ONSTACK if on_sig_stack(), see below.
> + onsigstack = on_sig_stack(sp); > + if (ss_size == 0) { > + switch (ss_flags) { > + case 0: > + error = -EPERM; > + if (onsigstack) > + goto out; > + current->sas_ss_sp = 0; > + current->sas_ss_size = 0; > + current->sas_ss_flags = SS_DISABLE; > + break; > + case SS_ONSTACK: > + /* re-enable previously disabled sas */ > + error = -EINVAL; > + if (current->sas_ss_size == 0) > + goto out; > + break; > + default: > + break; > + }
and iiuc the "default" case allows you to write SS_DISABLE into ->sas_ss_flags even if on_sig_stack().
So the sequence is
// running on alt stack
sigaltstack(SS_DISABLE);
temporary_run_on_another_stack();
sigaltstack(SS_ONSTACK);
and SS_DISABLE saves us from another SA_ONSTACK signal, right?
But afaics it can only help after we change the stack. Suppose that SA_ONSTACK signal comess before temporary_run_on_another_stack(). get_sigframe() should be fine after your changes (afaics), it won't pick the alt stack after SS_DISABLE.
However, unless I missed something save_altstack_ex() will record SS_ONSTACK in uc_stack->ss_flags, and after return from signal handler restore_altstack() will enable alt stack again?
Oleg.
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